As improvements continually have been made in television receivers, the number of adjustments which must be made by the viewer have been substantially reduced. One adjustment, however, which still remains in most receivers is a fine tuning adjustment. Such an adjustment is required even with receivers having automatic fine tuning (AFT or AFC) systems in them. With respect to the VHF channels, the fine tuning adjustment generally is made only when the receiver is first put into operation and then infrequently afterwards as components of the receiver age. For UHF channels, however, a fine tuning adjustment generally is required each time the UHF station is tuned in by the viewer. This is annoying and it is desirable to eliminate the need to such a fine tuning adjustment.
Copending application Ser. No. 537,692, filed Dec. 31, 1974, now U.S. Pat. No. 3,949,158, issued Apr. 6, 1976, is directed to a wide band AFT system which utilizes digital logic circuitry to extend the automatic control range of an AFT system by as much as .+-.2 megahertz. Such a system substantially minimizes, and in many cases eliminates, the need for fine tuning adjustments of television receivers which are rough-tuned by conventional detent-type turret tuners and the like.
It is desirable to employ channel selection systems in television receivers which permit direct selection of channels without the necessity of tuning through unused or unwanted channels to arrive at the desired channel. Many techniques have been suggested for accomplishing this. Most such direct select tuning systems employ a push button keyboard of the type commonly found in hand-held calculators or push button telephones to select the channel numbers. Decoding logic then is employed to change the keyboard information for selecting the channel into a form which effects the desired tuning of the receiver.
An ideal system for converting keyboarded direct select channel information into a usable control signal for tuning the receiver is a frequency synthesizer tuning system. Generally, this is accomplished by employing a programmable frequency divider between the output of the local oscillator or tuning oscillator of the receiver and one input to a phase comparator. The other input to the phase comparator is obtained from the output of a reference oscillator; and the output of the phase comparator comprises a tuning voltage which is used to control the frequency of the local oscillator. The division ratio of the programmable frequency divider is selected directly by the channel selection keyboard. Theoretically, this type of system is ideal for eliminating the need for fine tuning adjustments of a television receiver, so long as the reference oscillator is a highly stable oscillator. But even with a highly stable reference oscillator, frequency synthesizer systems fail to maintain proper tuning of television receivers in all cases, primarily because the signals from transmitting stations are not precisely maintained at the proper frequencies.
Thus, even with frequency synthesizer systems, it is necessary to employ an AFT or AFC circuit to take care of minor mistuning variations. For the most part, a conventional AFT system having a .+-.1 mHz pull-in range will be sufficient for a frequency synthesizer tuning system. In master antenna TV distribution systems, however, the UHF channels, and in some instances the UHF channels, are translated to an unused VHF channel. Such distribution systems are widely used, primarily in large cities, and particularly in hotel and motel installations. When inexpensive equipment is used for the translation, the resultant carrier may be mistuned a significant amount from the proper frequency. The amount of mistuning can easily be in excess of .+-.1 megahertz, the normal pull-in range of a good AFT or AFC system. It can even exceed .+-.2 megahertz. In the case where a conventional frequency synthesizer tuning system is used, a frequency off-set of this magnitude in a master antenna distribution system, or in any other case, will result in the mistuning of the received station; and the customer has no way of adjusting the RF oscillator for the mistuning.
It is desirable to automatically correct for frequency offsets in a frequency synthesizer tuning system without affecting the operation of the conventional frequency synthesizer in the tuning system. If this can be accomplished, the obvious advantages of frequency synthesizers in a television tuning system can be realized without the attendant disadvantages which otherwise exist when there is a frequency offset in the signal of the station to which the receiver is being tuned, whether such offset is created by a master antenna TV distribution or results from some other cause.